1. Field of the Invention
The present invention relates to a dust removal apparatus and an image pickup apparatus. In particular, the invention relates to a dust removal apparatus that removes a foreign substance such as dust attached onto a surface of an optical part that is incorporated in an image pickup apparatus such as a digital camera and an image reading apparatus such as a scanner by way of vibration and an image pickup apparatus.
2. Description of the Related Art
In an image pickup apparatus such as a digital camera that picks up an image by converting an image signal into an electric signal, an image pickup element such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) receives an image pickup light beam. Subsequently, a photoelectric conversion signal output from the image pickup element is converted into image data and recorded in a recording medium such as a memory card. A cover glass, an optical low-pass filter, an infrared absorption filter, or the like is arranged in an anterior stage (object side) of the image pickup element in the above-described image pickup apparatus.
When a foreign substance such as dust is attached onto a surface of the cover glass or the filter of these image pickup elements in the image pickup apparatus of this type, incident beam is shielded by the foreign substance and imaged as a black dot on a picked-up image in some cases. In particular, in a digital single lens reflex camera in which a lens can be replaced, dust is generated at the time of an operation of a mechanically operating part that is arranged in the vicinity of an image pickup unit such as a shutter or a quick return mirror or dust intrudes from an opening of a lens mount when the lens is replaced, and the dust may be attached onto the surface of the cover glass or the filter of these image pickup elements in some cases. In addition, the foreign substance such as the dust intrudes into a camera main body from the opening of the lens mount when the lens is replaced, and this dust may be attached in some cases. In view of the above, Japanese Patent No. 4790056 and Japanese Patent No. 5089065 disclose a dust removal apparatus including a piezoelectric element provided to the cover glass or the filter of these image pickup elements, and an elastic vibration is performed on the cover glass or the filter of these image pickup elements in a thick direction (hereinafter, will be referred to as flexural vibration) to remove the dust attached onto the surface, as well as an image pickup apparatus and an image reading apparatus including the dust removal apparatus.
Japanese Patent No. 4790056 and Japanese Patent No. 5089065 disclose the apparatus in which the piezoelectric element is provided outside an optical effective area of a rectangular optical film (an optical low-pass filter or an infrared absorption filter) and also along an end part so as to remove the dust attached onto the surface in the optical effective area by way of vibration. Japanese Patent No. 4790056 proposes a technology with which a dust control film that transmits the image pickup light beam is provided on an object side of the image pickup element, and this dust control film is vibrated by the piezoelectric element, so that the foreign substance such as the dust attached onto the surface of the dust control film is removed.
In a dust removal apparatus provided in a single lens reflex camera, a standing-wave bending vibration is generated by a driving frequency of a piezoelectric material. For example, when the piezoelectric is driven, the bending vibration is generated in a vibrating plate, and dust attached onto a surface of the vibrating plate can be effectively removed.
Lead zirconate titanate (PZT) is used as the piezoelectric material used for the dust removal apparatus that removes the dust on the image pickup element surface. However, PZT contains lead in an A-site of a perovskite structure. For that reason, an influence of a lead component on an environment is viewed as a problem. To address this problem, a piezoelectric material and a piezoelectric element using a perovskite type metal oxide that does not contain lead are proposed. For example, Japanese Patent Laid-Open No. 2008-150247 discloses barium titanate (BaTiO3) as the piezoelectric material formed by a perovskite type oxide that does not contain lead.
The piezoelectric element provided in the dust removal apparatus is driven when an alternating voltage having a frequency higher than a human audible range (a frequency between 20 Hz and 20 kHz) is applied to the piezoelectric element. Vibrations at the time of the driving include a vibration for removing the dust (main vibration) and a vibration for disturbing a vibration distribution of the main vibration in a frequency range close to its resonant frequency (undesired vibration). Since a vibration mode of the main vibration is determined by a frequency of the applied voltage, sound audible to a user is not generated. On the other hand, a plurality of undesired vibrations interfere with each other in various manners, and a vibration in the audible range is generated, and the user may hear this sound as noise in some cases. The undesired vibrations are generated by various causes. For example, the main vibration is incident on an end part of the vibrating plate, a reflected wave is generated from the end part. Accordingly, the incident wave and the reflected wave interfere with each other to generate the undesired vibrations. In addition, depending on dimensions of a piezoelectric body or an electrode pattern, a vibration generated in a direction perpendicular to the main vibration may be actualized as the undesired vibration in some cases.
In the dust removal apparatus, the bending vibration generated in the vibrating plate is designed by taking into account the dimensions of the piezoelectric element and the vibrating plate (length, width, and thickness) and the material physical properties (Young's modulus, density, and Poisson's ratio). In a case where the dust removal apparatus is used under a particular environment, it is also possible to construct the dust removal apparatus having a configuration where an influence of the undesired vibration does not exist without generating the audible sound.
However, a non-lead piezoelectric material such as BaTiO3 often has a phase transition temperature in an operating temperature range (between −30° C. and 50° C.), and physical property values of the piezoelectric material represented by Young's modulus largely change depending on its service temperature environment. For that reason, a machine design in which the undesired vibration is not generated in the entire operating temperature range is difficult, and a problem occurs that the large undesired vibration and the noise in the audible range (audible sound) derived from the undesired vibrations are generated depending on the temperature.
The present invention has been made in view of the above-described problem and provides a dust removal apparatus that has an excellent dust removal performance and also avoids generation of audible sound and an image pickup apparatus that can pick up a satisfactory image by using the dust removal apparatus.